Photocurable polymers and compositions are well known in the art for forming printing plates and other photosensitive or radiation sensitive articles. Generally, photocured printing plate comprise a support and a photosensitive surface or layer of photocurable composition. To prepare the printing plate the photosensitive surface is exposed to radiation in an imagewise fashion. The unexposed areas of the surface are then removed in developer baths.
In the past, removal of the unexposed surfaces has required the use of organic solvents which are environmentally unsafe, toxic and highly volatile. Thus, there has been a need to develop photocurable compositions which can be developed in non-organic solvent. In addition to possessing an aqueous photodevelopable photosensitive surface, a flexographic printing plate must be sufficiently flexible to wrap around a printing cylinder, while being strong enough to withstand the rigors of the typical printing process. Furthermore, the printing plate must be soft enough to facilitate ink transfer during printing and must be solvent resistant to inks typically used in printing. U.S. Pat. No. 5,328,805, Huynh-Tran, et al., "Aqueous Developable Photosensitive Polyurethane-(Meth)acrylate" (1994) and U.S. patent No. Huynh-Tran, et al., "Aqueous Developable Photosensitive Polyurethane-(Meth)acrylate" (1994) teach photopolymer resins comprised of a urethane prepolymer prepared by reacting polyoxyalkylene diols with polyester diols, or a mixture thereof, with an excess of diisocyanate followed by chain extending the resulting prepolymer mixture with an alkyldialkanolamine, then reacting the chain-extended product with a hydroxyalkyl(meth)acrylate.
However, improved mechanical properties such as resistance to inks including oil, water and alcohol based inks is still desired. Incorporation of butadiene would yield more flexible, softer compositions. However, until now, urethane prepolymers have not incorporated butadiene because butadiene polymers are generally incompatible with typical urethanes resulting in a cloudy product which causes light scattering during the imagewise exposure step. Light scattering results in poor image quality in the resulting printing plate.
The printing plates of the present invention overcome this limitation, incorporating butadiene into urethane prepolymers and resulting in photopolymer products which are more resistant to inks, have improved resilience, improved cold-flow properties and lower surface tension.